Urea Injection SCR for NOx Reduction in Hot Gas Streams
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Solution Overview
Problem
Existing NOx reduction methods in boilers, such as two-stage combustion and selective catalytic reduction (SCR) using anhydrous ammonia, face challenges including reduced boiler performance, safety hazards, increased energy consumption, and system complexity, particularly in sensitive environments like hospitals and schools.
Innovation Solution
A boiler assembly with multiple injectors for reducing agent delivery downstream of the furnace and flue gas outlet, combined with a selective catalytic reduction (SCR) catalyst, allowing simultaneous or downstream placement to optimize NOx reduction while minimizing adverse effects on boiler performance and safety.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If anhydrous ammonia is used as the ammonia source in SCR applications, then the NOx reduction effectiveness is improved, but significant safety hazards and health risks are introduced
Solution Approach 1:
Urea is used as an intermediary substance that decomposes to form ammonia in-situ within the boiler system. Instead of directly introducing anhydrous ammonia, the system introduces urea which then serves as a precursor, eliminating the need to handle and store hazardous anhydrous ammonia while still providing the necessary ammonia for NOx reduction through catalytic conversion
Solution Approach 2:
The patent employs urea as a readily available, low-cost reducing agent that can be easily stored and handled compared to anhydrous ammonia. Urea solutions can be stored in simple tanks without specialized safety infrastructure, and the system uses continuous injection of fresh urea solution rather than requiring long-term storage of hazardous materials
2Object-affected harmful factors
If external heat sources and conversion methods are used to convert urea to ammonia external to the boiler, then the safety hazards are reduced, but the system complexity and cost increase
Solution Approach 1:
The patent combines the urea decomposition function with the existing boiler heat field by injecting urea solution directly into the high-temperature flue gas stream. The boiler's own thermal energy is utilized to decompose urea into ammonia, eliminating the need for separate external heating systems, conversion vessels, and associated complex infrastructure
Solution Approach 2:
The boiler system serves its own dual purpose: it not only generates steam or heat but also provides the thermal environment necessary to decompose the urea reducing agent into ammonia. The flue gas temperature and flow within the boiler automatically facilitate the urea decomposition and ammonia release process without requiring external service systems
3Ease of operation
If external ducting with heat tracing and insulation is used to deliver ammonia to the flue gas, then the ammonia delivery is controlled, but the energy consumption and maintenance requirements increase
Solution Approach 1:
The patent extracts the ammonia generation function from the external delivery system and relocates it directly into the boiler flue gas stream. Instead of delivering pre-formed ammonia through complex ducting systems, the system generates ammonia in-situ within the high-temperature environment of the boiler, eliminating the need for external delivery infrastructure
Solution Approach 2:
The patent replaces the mechanical delivery system (ducts, pumps, tracing, insulation) with a chemical decomposition approach. Urea solution is injected directly into the flue gas, and the thermal energy of the gas stream itself drives the decomposition reaction, substituting mechanical delivery control with thermal-field-driven chemical conversion
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach effectively reduces NOx emissions with minimal impact on boiler performance and safety, reducing energy consumption and system complexity, making it suitable for sensitive environments.
Implementation Method 1
selective catalytic reduction (SCR) for NOx reduction
Implementation Method 2
reducing urea to ammonia to avoid some of the health risks associated with storing large amounts of anhydrous ammonia
Data Source
AI summary
A boiler or other fired vessel includes a housing with a burner at one end, a furnace downstream of the burner, a convection section downstream of the furnace and a flue gas outlet downstream of the convection section. A first means for loading a reducing agent comprising at least two injectors is located downstream of the furnace. A second means for loading a reducing agent is located downstream of the first means for loading a reducing agent. A selective catalytic reduction catalyst is located either downstream of the second means for loading a reducing agent or adjacent the second means for loading a reducing agent such that the catalyst is provided to the boiler or other fired vessel approximately simultaneously with the reducing agent from the second means for loading the reducing agent.